An important criterion in the performance of computed tomography (CT) is the ability to image a large volume in a short time period. This allows for reduced artifacts caused by patient movement and for higher patient throughput.
One method of improving throughput is to utilize a helical scan. In such a scan, an x-ray source that irradiates a patient rotates about the patient in a helical trajectory. Attenuation data is acquired along the entire trajectory. This data is used to reconstruct “slices” of cross-sectional attenuation images perpendicular to the axis of the helix.
Another method, sometimes used in conjunction with the helix scan, provides a plurality of axially displaced rows of circumferentially spaced detectors. Scanners embodying this method are sometimes called “multi-slice” scanners. Generally, a single source is used to irradiate the patient (and the multiple slices within the patient), such that the patient is irradiated by a “cone beam”. At least some of the detectors detect attenuation data that does not represent attenuation through a slice perpendicular to the axis. Since the standard reconstruction algorithms for CT are based on axial slices, multi-slice methods intrinsically utilize inconsistent data in reconstructing the slices transverse to the axis. However, so long as the angle of the radiation is relatively low, this effect can be neglected without causing substantial artifacts. As the angle of the cone beam increases, so do artifacts. Generally, special cone beam reconstruction algorithms are used when the angle is great enough to cause bothersome artifacts.
Cone beam reconstruction algorithms have been developed which take account of the non-transverse nature of the data traces. Representative examples of such algorithms are taught in “A General Cone-Beam Reconstruction Algorithm” by Ge Wang, et al. IEEE Trans. Med. Imag. Vol. 12, No. 3, September 1993, Pages 486–496 and “Advanced single-slice rebinning in cone-beam spiral CT” by Marc Kachelrieβ, et al, Med. Phys. Vol. 27, No. 4, April 2000, pages 754–772.
One of the features of reconstructing attenuation maps from cone beam (as well as other helical) acquisition methods is that the length of the helix described by the x-ray source is larger than the length of the region being scanned. This additional length is required since, in order to obtain full coverage of the length required, some of the non-transverse rays at the beginning and end of the scan are not usable.
There exist methods of reducing the length of said scans. WO 99/01736 describes a method in which the helix axis and the patient axis are coincident and the plane formed by the axis of the central row of detectors and the X-ray source is tilted with respect to the joint patient/helix axis. This is said to improve the efficiency of use of the detector array.
In general, shortening of the helical path results in the generation of artifacts originating from the conical geometry of the beams. Shortening of the helical path can not go beyond a limit determined by the conical reconstruction method without causing additional artifacts.
U.S. Pat. No. 4,989,142 and EP patent publication EP 0 981 996 describe methods for converting data in systems in which the patient and helix axis are different to transaxial patient data.
In some CT scanners, the axis of the patient (i.e., the axis along which the bed carrying the patient moves, during the scan) can be angled with respect to the axis of rotation of the CT scanner and the acquired “axial” slices (which are transaxial with respect to the helix axis). As used herein, unless otherwise specified, the term “axis” or “axial” refers to the axis of rotation of the CT scanner and not to that of the bed movement. In the prior art, such angling is generally provided so that transaxial slices are properly positioned with respect to the anatomy that is being imaged. Thus, such angling is generally between 7 and 15 degrees, since lesser changes do not generally improve visualization to a significant extent.